A typical optical communications system comprises a source of optical input signals, a network of optical waveguides coupled to the source, one or more optical amplifying devices along the waveguides and one or more optical receivers.
The waveguides are typically optical fibers--small diameter circular waveguides characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical telecommunications fibers are made of high purity silica with minor concentrations of dopants to control the indices of refraction. They can transmit an optical signal containing a large amount of information.
For long distance applications, such as undersea cable and many terrestrial applications, it is necessary to periodically amplify the transmitted signal as by passage through a rare-earth doped fiber amplifier. Such amplifiers typically comprise a short (typically 30 m) length of optical fiber doped with a small percentage of rare earth elements such as Er. After these fibers are exposed to light of an appropriate pump wavelength (shorter than the transmitted wavelength) they are capable of stimulated emission at the signal wavelength. Typical communications systems utilize signal wavelengths of about 1550 nm and Er fiber amplifiers pumped at 980 or 1440 nm.
Amplifying arrangements with compact, efficient pumping are currently a problem in optical communications. In general, the higher the level of pumping power, the greater the amplification. Raman amplification, for example, requires much more pump power than can be supplied by single semiconductor laser pumps. Arrays of pumps are used to supply adequate pump power. Anticipated applications seek as much pumping power as can be compactly generated and efficiently injected into the amplifier cores. The conventional pumping sources are arrays of semiconductor laser diodes. But the output of each diode is limited and it is difficult to combine the outputs of numerous diodes for efficient injection into the tiny core of an amplifying fiber.
U.S. Pat. No. 5,268,978 describes an apparatus for combining the outputs of numerous diodes. In essence, a plurality of fibers have one set of ends horizontally aligned along a cylindrical lens in registration with the diodes and the other ends are vertically stacked and focused onto a fiber laser having a large core. The difficulty with this approach for communications applications is that precise assembly and alignment of the numerous small components used in this apparatus would be prohibitively expensive and time-consuming. Moreover the large core multimode lasers thus produced are unduly large for communications applications.
Accordingly there is a need for an improved apparatus for pumping an optical fiber laser.